Considering the low price and the availability of a ROPS from Kubota I would say that was the best route to take, However if one was not available or if the price was prohibitive and you are a proficient welder then I see no problem with fabricating your own as even limited roll over protection is better than none at all, Personally I am not inclined to put my tractor into any situation where I would feel safer with than without a ROPS, In other words just because I have a ROPS does not mean I am more inclined to do something that would risk rolling my tractor over on its side.
The ROPS is designed to protect the driver in a side roll over not front to back roll over and is designed to be used in conjunction with a seat belt.
My tractor has a warning sticker stating that the seat belt should be used at all times on a tractor equipped with a ROPS and a seat belt should not be used when the tractor is not equipped with a ROPS or in situations wher the ROPS is not being used(Folded down). In other words if you have a ROPS you should be strapped in to keep you within the protective parameters of the ROPS however if you have no ROPS then you do not want to be strapped into the seat as you want to be able to eject yourself from the tractor befor it rolls over on you.
So if you are installing a ROPS do you have or are you also installing a seat belt?
Does the ROPS kit from Kubota also include a seat belt kit?
I just checked and a seat belt does come with the ROPS kit from Kubota